1. Field of the Invention
The present invention relates in general to the substrate structure of a monolithic gas sensor. In particular, the invention relates to the substrate structure better thermal stress characteristics that can be fabricated in simple semiconductor fabrication procedural steps.
2. Technical Background
Micromachining techniques have been advancing at a rapid rate for the last several decades, especially in the category of devices employing single crystal silicon for substrate material. One of the major applications of such techniques is in the fabrication of microsensors. Fabrication techniques of these microsensors are very similar to that for microelectronic devices. Batch manufacturing of these microsensors are widely employed, with relatively much lower costs than for the production of many conventional single-element sensing elements.
One important application of these sensing elements is the use as sensors for the existence of poisonous gases. Specific material (or materials) reacts with certain gases hazardous to human health,in order to signify the existence of such gases. Based on such detected reactions, the presence of such hazardous gases may also be expressed in quantity. There are quite many such sensing elements, such as those disclosed in U.S. Pat. Nos. 4,198,950; 4,224,280;and 4,792,433 for carbon monoxide. Among them, those employing tin oxide as sensor material and implemented as sensing elements which can be seen in markets are shown in FIGS. 5, 6 and 7.
The sensing element depicted in FIG. 5 is one that was developed earlier although still in use currently. The sensing element is shown in the drawing to be comprised of device base 1, ceramic tube 2, tin oxide film 3 coated over the surface of ceramic tube 2, heating resistor coil 4, electrode 5 and lead wires 6. All those portions seen in the drawing other than the device base 1 comprises the sensing element body. The ceramic tube 2 is heated to a temperature of about 300.degree. to 400.degree. C. when the sensing element is used. At this temperature, the specially fabricated tin oxide film 3 is particularly sensitive so that it would be reacting with certain reducing gases. When such reactions take place, electrical resistance in the film would change (decrease under normal circumstances). Lead wires 6 provided at both ends of the device may therefore be connected to an external circuitry for conducting measurements on the extent of resistance change. This allows for the quantification, in addition to the detection, of those gases suspected to be present.
In the structural configuration schematically depicted in FIG. 5, physical size of the sensing element is relatively bulky. Further, due to the fact that ceramic tube 2 and heating coil 4 are widely separated apart from each other, considerable electrical power therefore has to be consumed before the temperature can be heated up to several hundred degrees Celsius. In addition, sensing element heated to high temperature must be separated from the supporting base 1 utilizing suspension means such as inactive platinum strings having high melting temperature. This substantially avoids direct physical contact between body of the sensing element and the supporting device base 1, so that thermal leakage from heated sensor body to a base 1 can be prevented. Since this type of gas sensors have a structure that is manufactured as discrete units, specially-designed automated manufacturing facilities must be used for production, costs are therefore high.
FIG. 6 shows the partially cut-away perspective view of a conventional gas sensing element developed in recent years. The sensing element is shown in the drawing with an enlarged detail of the thick film gas sensor used therein. As can be seen, the sensing element is also comprised of a sensing element body and the device supporting base 1. Essentially, the sensing element body consists of an aluminum oxide ceramic base 7, heating resistor 10, gold-plated electrodes 8 and the tin oxide sensor layer 9. Heating resistor 10 may be made, for example, of RuO.sub.2 thick-film resistor. This sensor structure employs aluminum oxide ceramic as the base material that replaces the ceramic tube in the element of FIG. 5. Thick film screen printing technique is utilized to manufacture the heating resistor 10, sensor layer material 9 and electrodes 8 at both sides of the sensing element body. Similar structures had been disclosed in quite many U.S. patents, such as U.S. Pat. Nos. 4,792,433; 4,345,985; 4,224,280; 4,885,929 and 4,198,850. Although such sensing element body can be batch produced based on thick film processing technology, however, suspension structure is still required when the sensing element body is to be sealed in the device carrier. This is for the same reason of thermal isolation between the sensor body and the device base 1. Different kinds of supporting structures for good thermal isolation have been patented, such as those disclosed in U.S. Pat. Nos. 4,596,975 and 4,656,863. However, in these disclosures sealing remains to be the high cost portion for the manufacture of these single-unit sensing elements. Moreover, substrate surface area of these sensing elements remains to be relatively large as the screen printing technique has limited precision capability which contributes directly to the excessive amount of gas conductive heat loss in the surrounding atmosphere. Typical thermal power consumption is in the range of several hundred milli-watts. For a sensor system operating on battery power, this power consumption rating is barely practical.
Gas sensing elements based on micromachining fabrication technology have recently been patented and/or commercialized. An example is the device structure depicted in FIG. 7 as manufactured by Motorola in U.S. Pat. No. 4,706,493 by Chang et al. The cross-sectional view schematically depicted in the drawing shows that the device is fabricated utilizing a single crystal silicon substrate 11 as basis. A silicon nitride or oxide layer 12 is formed on the surface of the substrate 11 as an insulating layer, with the heating resistor 13 for the device embedded therein. A tin oxide sensor layer 14 is formed on the surface of the insulating layer 12 which is connected to external circuitry via a pair of contact electrodes 15. This is a structural configuration employing anisotropic back-side etching technique to form the thin insulating layer 12 substantially suspended above the surface of (100) silicon single crystal substrate 11. On top of the thin layer 12, heating resistor 13 and sensor film 14 may be formed by photolithographic technique.
In this depicted Motorola configuration, a heavily-boron-doped epitaxial silicon region 111 is formed in the substrate 11 underneath the thin insulating layer 12. Existence of this region serves to suppress the etching consumption to the region having a boron concentration of about 10.sup.19 -10.sup.20 /cm.sup.3. This allows for formation of the layer with predetermined thickness (i.e., the heavily-boron-doped epitaxial silicon layer 111). On the other hand, since a sensing element is subject to great structural distortion under high temperature as a result of excessive thermal stress. However, since boron-doped epitaxial silicon layer 111 has a thermal expansion coefficient substantially the same as that of the other portions of the silicon substrate 11, therefore, damage to the overall substrate structure can be prevented by sharing to withstand the stress produced internal to the insulating layer 12.
Sensor element shown in FIG. 7 has an overall structural surface area of about several tens of square milli-meters. Joule heat leakage to the supporting base 1 seen in FIG. 8 as produced by the thin layer 12 that amounts to solid conductive loss is generally transferred along the direction as schematically expressed in FIG. 7 by phantom arrow-headed paths. The heat is transferred from the thin layer 12 to the un-etched silicon substrate supporting pads 112. Since silicon is a good heat conductor, silicon substrate supporting pads 112 may therefore have low thermal resistance characteristics. As a result, the temperature difference between the high-temperature sensor layer 14 and the room temperature supporting base falls on the thin layer 12. The silicon supporting pads 112 may thus be stabilized at the temperature close to that of the supporting base. The monolithic gas sensor can therefore be directly bonded to the supporting base having good thermal conductivity characteristics without increased thermal loss, and the supporting base may be kept at relatively normal temperature. Sealing for this type of sensing element does not require the conventional suspension mechanism (such as those employed for the sensing elements depicted in FIGS. 5 and 6) in order to isolate hear transfer. Instead, TO metal-can packaging for conventional microelectronic circuit devices can be used in a simple and easy manner. The cross-sectional view of FIG. 8 depicts an example of this type of packaging. Since standardized packaging material and automated packaging equipments are readily available, costs can therefore be greatly reduced. Meanwhile, traditional suspension packaging mechanisms described above employing fine and long platinum strings for soldered suspension would require lower lead wire thermal loss and higher mechanical strength characteristics. In comparison, monolithic gas sensors fabricated out of micromachining technique have lead wires thereof placed and soldered to the top of the silicon supporting pads. This top portion of the silicon supporting pads has a temperature close to that of the silicon pad, there is therefore no concern of thermal loss via the conduction of the soldered lead wires. In addition, weight of the sensor chip is supported directly by the entire support base 1, the problem of mechanical strength of the wires is practically non-existent. As a result, standard and cheap gold bonding wires may be used for electrical connection. In other words, utilizing gold bonding wires in standardized and automated bonding operation of the sensing elements would incur much less manual intervention as well as much lower costs than when platinum strings are used.
Gas sensing elements implemented in the form as single-chip units may thus enjoy many advantages than their traditional counterparts. In summary, these advantages may be compared in the following Table 1.
TABLE 1 ______________________________________ Ceramic Tube Thick Film Single-chip ______________________________________ Packaging Structure .circle-solid.Suspended Suspended Placed and Production .circle-solid.Single-unit Single-unit bonded production production Batch production, .circle-solid.Difficult to Difficult to auto- Standardized and automate mate automated fast Slow Produc- Slow production production tion Material .circle-solid.Platinum Platinum Gold bonding suspension suspension wire, wires, low cost wire, expensive expensive .circle-solid.Non-standard Non-standard Standard TO-can supporting base supporting base supporting base .circle-solid.Requiring Requiring special Easy to acquire special custom custom design and low in cost design Sensor Element Size Large Medium Small Current Large Medium Small consumption (About 1A) (about 200 mA) (50 mA or smaller) Fabrication Single-unit Thick-film Micromachining process production batch production batch production (Slow) (Fast) (Fast) Precision Low Medium High Cost High Medium Low Battery Not practical Difficult feasible operation Circuitry Not Possible Not Possible feasible integration ______________________________________
From the comparisons outlined in Table 1 above, the monolithic gas sensor of FIG. 7, although superior than the conventional ones by at least the listed advantages, still suffers the following drawbacks. Specifically, with reference to FIG. 8, since silicon is a material of good thermal conduction, with a thermal conduction coefficient about 120 times that of silicon oxide, and about several tens of times that of silicon nitride, therefore, heat in high temperature region of the thin insulating layer 12 can be transferred to the supporting base 1 in a fast rate via the heavily-doped epitaxial silicon layer 111 directly underneath the layer 12. Thus, the problem of thermal power loss in such sensing elements is still unsolved.